In the start-up of a new or rebuilt furnace for melting glass in a glass fiberizing process it is necessary to run the furnace for a period of time until the melting process has stabilized and impurities from the construction, such as particles of refractory material, have been flushed from the system. The molten glass in the system during this period is drained through drain bushings, which are electrically heated to very high temperatures and which typically have two or three large drain holes as opposed to the large number of very small apertures in the fiberizing bushings used in the production of glass filaments.
The fiberizing bushings are normally formed of precious metal, such as platinum/rhodium alloys, which are very expensive due to the cost of the precious metal and the cost of fabrication. Drain bushings also have been formed of precious metal, at consequent high cost. Since drain bushings are used only for relatively short periods of time, in the order of five to ten days, it is not necessary that they have a long life. This in fact is detrimental from a cost standpoint because it results in an unnecessarily large amount of precious metal being tied up in drain bushings which are used only infrequently. It would be better to use drain bushings formed from non-precious metal, provided they are able to adequately withstand erosion from the molten glass until the draining operation is over, and provided they can be adequately heated to the required temperatures.
The usual vertically arranged relatively thick lug-type terminal typically used with precious metal bushings has not been found to allow satisfactory electrical heating of a non-precious metal drain bushing. The heat is not dispersed uniformly throughout the bushing, resulting in hot spots which are deleterious to the life of the bushing and which do not promote stable operating conditions. Other types of conventional terminals, such as the wider wing-ear style, have not been found to be satisfactory either. The desired heating patterns are still difficult to achieve and the terminal ears, when made thin enough to obtain better heat transmission, display a tendency to bend when subjected to the pressures caused by the power clamp over a period of time.